1. Field of the Invention
The present invention relates to a ferroelectric thin film, a manufacturing method thereof and a device incorporating the same. More particularly, it relates to a ferroelectric thin film, a manufacturing method thereof and a device incorporating the same which can be applied to a ferroelectric memory element, a pyroelectric sensor, a piezoelectric element and the like.
2. Description of the Related Arts
Having numerous properties such as spontaneous polarization, high dielectric constant, electrooptical effect, piezoelectric effect, pyroelectric effect and the like, a ferroelectric is applied to a wide range of devices such as capacitors, oscillators, optical modulators, infrared sensors and the like.
Recently, by applying the spontaneous polarization property of the ferroelectric to a memory, a ferroelectric non-volatile memory has been realized which shows a great improvement in operation speed and data rewriting time as compared with a conventional non-volatile memory such as an EEPROM or flash memory. Also, by utilizing the high dielectric constant property, the capacitor size is made smaller, whereby high integration of semiconductor elements such as DRAMs is achieved, and a device of giga bit size is fabricated on an experimental basis.
As described above, it is essential to develop a technique for reducing the thickness of a ferroelectric film in conformity with the conventional semiconductor processes in order to apply the ferroelectric to various devices such as semiconductor elements. In other words, it is desired to develop ferroelectric materials which can realize a desired property with a small thickness produced by lowering the film-forming temperature and by making a dense and flat film whereby the ferroelectric thin film is suitable for fine processing and reduction of operation voltage. Also, it is desired to develop a technique for reducing the thickness of ferroelectric films.
Conventionally, lead titanate zirconate (Pb(Zr.sub.1-X Ti.sub.X)O.sub.3 ; PZT) has been widely used as the ferroelectric material. However, PZT accompanies a problem that deterioration of ferroelectric property (film fatigue) generated by repetition of polarization inversion is large.
Among bismuth-based layered compounds represented by the formula: Bi.sub.2 A.sub.m-1 B.sub.m O.sub.3m+3 (wherein A is at least one element selected from the group consisting of Na, K, Pb, Ca, Sr, Ba, and Bi; and B is at least one element selected from the group consisting of Fe, Ti, Nb, Ta, W and Mo), ferroelectric materials such as bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.12), strontium bismuth tantalate niobate (SrBi.sub.2 (Ta.sub.1-X Nb.sub.X).sub.2 O.sub.9) (0.ltoreq.X.ltoreq.1) and the like are attracting public attention because of their resistance to film fatigue, and has been widely developed in a technique for reducing the film thickness.
Especially among these ferroelectric materials, Bi.sub.4 Ti.sub.3 O.sub.12 (rhombic system, lattice constants: a=5.4100.ANG., b=5.4489.ANG., c=32.815.ANG. (JCPDS data card 35-795)) is a material showing a ferroelectric property with a strong anisotropy. A bulk ferroelectric property of the material shows a spontaneous polarization of 50 .mu.C/cm.sup.2 and a coercive field of 50 kV/cm along the a-axis. The spontaneous polarization of the material is the largest among the above bismuth-based layered compounds. In contrast, the material has a very small coercive field of 4 kV/cm along the c-axis although the spontaneous polarization along the c-axis is as small as 4 .mu.C/cm.sup.2.
A Bi.sub.4 Ti.sub.3 O.sub.12 film is formed with a small thickness by a number of methods such as a MOCVD method, a sol-gel method, a sputtering method and the like.
A lot of reports on the sol-gel method are provided because of its facility in film-forming control. However, the sol-gel method generally involves high temperature thermal treatment of 650.degree. C. or more with the grain size being as large as about 0.5 .mu.m, so that it is difficult to obtain a good property with a film thickness of 200 nm or less and application to fine processing is also difficult.
On the other hand, the MOCVD method is expected for application to a practical device processes because reduction of the film thickness of a large area is generally possible and the step coverage is good. However, reduction of the film thickness of Bi.sub.4 Ti.sub.3 O.sub.12 by a conventional MOCVD method is carried out at a high substrate temperature of 600.degree. C. or more and, moreover, the obtained film is in most cases a c-axis oriented film having large crystal particles. Therefore, it was not possible to obtain a thin film of Bi.sub.4 Ti.sub.3 O.sub.12 having a large polarization component along the a-axis.
Further, a three-step growth method is recently proposed by the inventors of the present invention (See Japanese Unexamined Patent Publication No. HEI 8(1996)-306231). According to the method, growth of main Bi.sub.4 Ti.sub.3 O.sub.12 thin film to be formed on a buffer layer is achieved at a low substrate temperature of 400.degree. C. by using a double buffer structure in which a very thin titanium oxide buffer layer is formed on a substrate at a substrate temperature of 400.degree. C. and, further, a very thin Bi.sub.4 Ti.sub.3 O.sub.12 film is laminated thereon at a substrate temperature of 400.degree. C. to 650.degree. C. The obtained thin film has superior crystallinity, density and surface flatness and is shown to have, in the case of a film thickness of 200 nm or less, a better ferroelectric property than those previously reported.
However, in addition to good ferroelectric property, simplification of the process is earnestly desired. Further, the thin film of SrBi.sub.2 (Ta.sub.1-X Nb.sub.X).sub.2 O.sub.9 has a property that the film fatigue generated by the polarization inversion is very small although the remaining spontaneous polarization thereof is about 10 .mu.C/cm.sup.2 which is inferior to that of Bi.sub.4 Ti.sub.3 O.sub.12.
Reduction of the thickness of SrBi.sub.2 (Ta.sub.1-X Nb.sub.X).sub.2 O.sub.9 film is carried out mainly by a coating film-forming method such as the sol-gel method or a MOD method.
However, it is believed that a thermal treatment for the method needs a temperature of 800.degree. C. or more, so that it is difficult to apply the method to fabrication of semiconductor devices by itself. Namely, in the coating film-forming method, it is difficult to finely control the film thickness and also, since the method involves a high temperature process, reaction of the film with other materials constituting the device and the like occurs; and further, the grain size is large, so that the method is not suitable for fine processing. Therefore, reduction of the film-forming temperature is desired.
Generally, in order to apply a ferroelectric material to a semiconductor device, it is desired to suppress the reaction of a film with other materials constituting the device including an electrode material by lowering the film-forming temperature and to realize a dense thin film having a flat surface so as to obtain a good ferroelectric property with a small film thickness of 200 nm or less.
However, as described above, technique for obtaining a good ferroelectric property as well as realizing a fine processing and the like has not been achieved and is not fully applied to practical device development.